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What is grey water?

Any washwater that has been used in the home, except water from toilets,
is called grey water. Dish, shower, sink, and laundry water comprise
50-80% of residential "waste" water. This may be reused for
other purposes, especially landscape irrigation.

(This is the definition common in Europe and Australia. Some jurisdictions in the US exclude kitchen sink water and diaper wash water from their definition of grey water. These are most accurately defined as "dark grey water")

Why use grey water?

It's a waste to irrigate with great quantities of drinking water when
plants thrive on used water containing small bits of compost. Unlike
a lot of ecological stopgap measures, grey water reuse is a part of
the fundamental solution to many ecological problems and will probably
remain essentially unchanged in the distant future. The benefits of
grey water recycling include:

Lower fresh water use

Less strain on failing septic tank or treatment plant

Better treatment (topsoil is many times more effective than subsoil or treatment plant)

Less energy and chemical use

Groundwater recharge

Plant growth

Reclamation of otherwise wasted nutrients

Increased awareness of and sensitivity to natural cycles

Healthy fruit from sanitary irrigation of edible
landscape

Why does grey water matter?

Viewed narrowly, grey water systems don’t look that important. A low flow showerhead can save water with less effort. A septic system can treat grey water almost as well.
But when you look at the whole picture—how everything connects—the keystone importance of grey water is revealed.

Ecological systems design is about context, and integration between systems. The entirety of integrated, ecological design can be reduced to one sentence: do what's appropriate for the context.

Ecological systems—rainwater harvesting, runoff management, passive solar, composting toilets, edible landscaping—all of these are more context sensitive than their counterparts in conventional practice; that's most of what makes them more ecological.

Grey water systems are more context sensitive than any other man-made ecological system, and more connected to more other systems.

Get the grey water just right, and you’ve got the whole package right—and that matters.

Many people and organizations instinctively recognize that grey water is the ideal test case for the transition to a new way of regulating and building that is appropriate to a post-peak resource, mature civilization.

The US Green Building Council, the City of Santa Barbara, CA, Oregon ReCode, and SLO Green Build are among those organizations which independently chose grey water standards as the technology with which to launch their programs of regulatory reform.

Is grey water reuse safe?

Yes. There are eight million grey water systems in the US with 22 million users. In 60 years, there has been one billion system user-years of exposure, yet there has not been one documented case of grey water transmitted illness.

Is grey water legal?

In practice, grey water legality is virtually never an issue for residential retrofit systems—everyone just bootlegs them. However, grey water legality is almost always an issue for permitted new construction and remodeling, unless you're in a visionary state such as Arizona, New Mexico, Texas (and soon, NV, MT, OR, and CA). For details see our Grey water policy center and Builder's Grey Water Guide (book).

The benefits of grey water recycling (in detail)

* Lower fresh water use

Grey water can replace fresh water in many instances, saving money and increasing
the effective water supply in regions where irrigation is needed. Residential
water use is almost evenly split between indoor and outdoor. All except toilet
water could be recycled outdoors, achieving the same result with significantly
less water diverted from nature.

* Highly effective purification

Grey water is purified to a spectacularly high degree in the upper, most biologically
active region of the soil. This protects the quality of natural surface and
ground waters.

* Site unsuitable for a septic tank

For sites with slow soil percolation or other problems, a grey water system
can be a partial or complete substitute for a very costly, over-engineered system.

* Less energy and chemical use

Less energy and chemicals are used due to the reduced amount of both freshwater
and wastewater that needs pumping and treatment. For those providing their own
water or electricity, the advantage of a reduced burden on the infrastructure
is felt directly. Also, treating your wastewater in the soil under your own
fruit trees definitely encourages you to dump fewer toxic chemicals down the
drain.

* Groundwater recharge

Grey water application in excess of plant needs recharges groundwater.

* Plant growth

Grey water enables a landscape to flourish where water may not otherwise be
available to support much plant growth.

* Reclamation of otherwise wasted nutrients

Loss of nutrients through wastewater disposal in rivers or oceans is a subtle,
but highly significant form of erosion. Reclaiming nutrients in grey water helps
to maintain the fertility of the land.

* Increased awareness of and sensitivity to natural cycles

Grey water use yields the satisfaction of taking responsibility for the wise
husbandry of an important resource.